I have 2 cars that spec. 5W-20, one has 40,000 miles, and I have been useing Amsoil XL at 7500 mile oci. This car is off warranty, would Mobil 1 Extended Performance at 10,000 mile oci be a better choice. They are about the same price. My othe car is still under warranty with 7000 miles, so 5000 mile oci for this one, p.s. it is expected to get double digits below zero tonight, so i would like to use syn. oil in both. yor comments welcome Thanks